1. Field of the Invention
The subject invention relates to a laser alignment system which comprises a plurality of lasers, a target and a computer for controlling operation of the lasers and for analyzing signals produced by the target for determining spatial location and angular/slope data.
2. Description of the Related Art
Many manufacturing processes require the alignment of a plurality of parts that are spaced from one another. For example, a rotating tool may require a specified alignment to a workpiece for boring a hole in the workpiece or for taping threads into a previously bored hole. In other situations, arrays of rotors or stators must be precisely aligned to the axis of a turbine. In still other situations, sheaves or pulleys of a machine tool should be mounted to rotational axes that are precisely parallel to one another, at specified distances and with the sheaves in a common plane. Other manufacturing processes require parts to be assembled at specified positions relative to reference planes. For example, seats and storage bins on large aircraft should be accurately positioned relative to horizontal and vertically aligned planes extending along the length of the aircraft.
Historically, most of the above-described alignment has been carried out with purely mechanical devices, such as bubble levels, or by purely optical devices, such as sight gages. However, a very effective prior art apparatus for checking alignment includes a laser emitter and a photosensitive target. The laser emitter produces a perfectly straight beam that is not affected by gravity. The target is operative to generate signals that identify the center of energy of the laser beam impinging on the target. The target of the prior art laser alignment system typically is connected to a computer which calculates certain displacement and angular alignment information based on the signals produced by the target. This prior art system is used by mounting the laser to a fixed location and by mounting the target to a specified location on a part that must be aligned. A very simple but effective apparatus for aligning a rotating tool to a master part or workpiece is disclosed in U.S. Pat. No. 4,566,202 which is assigned to the assignee of the subject invention.
Some prior art laser apparatus include a penta-prism which is operative to receive an input laser beam and to reflect that input laser beam through precisely 90xc2x0. The penta-prism then may be rotated about the axis defined by the input laser beam. Thus, the output laser beam effectively sweeps a flat laser plane that is perpendicular to the input laser beam. The plane produced by the rotating laser defines a frame of reference. A plurality of targets then may be positioned in the reference plane, and the locations of those targets can be measured precisely relative to the reference plane. A laser apparatus with a rotating penta-prism for sweeping a flat optical plane is shown in U.S. Pat. No. 4,297,031 which is assigned to the assignee of the subject invention. U.S. Pat. No. 4,297,031 also shows the above-referenced plurality of photosensitive targets mounted at locations to be impinged upon by the rotating output laser.
The assignee of the subject invention also is the owner of U.S. Pat. No. 5,307,368 which is directed to an apparatus for simultaneously generating a plurality of mutually perpendicular planes. The device shown in U.S. Pat. No. 5,307,368 enables targets to be placed in each of the mutually perpendicular planes for positioning an object relative to those planes.
The above-described prior art laser alignment systems typically employ a plurality of targets for defining position and alignment relative to at least one plane swept by at least one rotating output laser beam. For example, a plurality of photosensitive target cells have been used in the prior art to assess both displacement and angular misalignment.
In recent years, low cost, low accuracy laser alignment systems have entered the market. The availability of a lower cost, albeit poorer quality, laser alignment system has created a market pressure to decrease the cost for the high quality laser alignment systems. During this same time span, the cost of photosensitive target cells has remained high, and in some instances has increased. Hence, the cost for photosensitive targets in a laser alignment system that requires plural targets limits the ability to respond to market pressures for lower cost alternatives without sacrificing accuracy or the quality of workmanship. Simultaneously, however, the cost of lasers has decreased significantly.
In view of the above, it is an object of the subject invention to provide a low cost, high quality laser alignment system.
Another object of the subject invention is to provide a laser alignment system that enables an assessment of displacement and angular alignment errors with a single target.
A further object of the subject invention is to provide a laser alignment system with plural laser beams and a single target for assessing displacement and angular alignment.
The subject invention is directed to a laser alignment system comprising a plurality of lasers and a light and position-sensitive target cell. A control system may be provided for turning the lasers on and off sequentially. Thus, the single light and position-sensitive target cell is operative to sequentially generate signals indicative of locations at which the respective laser beams impinge upon the target cell. The control system may further be connected to the target cell for receiving signals produced by the target cell and for analyzing the signals to provide position and alignment information.
The system of the subject invention may include first and second lasers and a beam splitter. The beam splitter may be positioned in proximity to the single light and position-sensitive target cell. In particular, the beam splitter may be disposed such that a first beam produced by the first laser passes through the beam splitter and impinges upon the target cell. As the target cell is moved relative to the first beam, vertical and horizontal position signals are generated. The second laser produces a second beam that is aligned generally parallel to the first beam. The system further includes a mirror disposed to align with the second beam. The mirror reflects the beam to the beam splitter which further reflects the beam onto the light and position-sensitive target cell. Additionally, the system may include a lens disposed between the source of the second beam and the mirror. Thus, the second beam is focused by the lens and is directed toward the mirror. The combined optical distance from the lens to the mirror, from the mirror to the beam splitter and from the beam splitter to the target cell is selected to substantially equal the focal length of the lens. Thus, the second beam will substantially focus on the single light and position-sensitive cell in the target assembly.
When the first laser is turned on, the first beam acts as a position measuring device that generates X-axis and Y-axis (horizontal and vertical) position signals to indicate displacement of the target assembly relative to the incoming first laser beam. When the second laser is turned on, the target acts as an angle or slope sensitive target in both pitch and yaw. Additional readings of the cell are taken a short time after each of the center or angle readings. These readings constitute background light readings and are subtracted from the respective readings when the laser is on. The result is a compensation for the background light that is also falling on the target cell.
An alternate embodiment of this invention includes a lens, a quarter wave plate, a mirror and first and second lasers. The first laser produces a first beam that passes straight through the lens. The lens starts to focus the beam on the cell. The second laser produces a second laser beam that is parallel to or off-axis relative to the first beam. The second beam passes through the lens, hits the mirror, reflects back to the polarizing beam splitter coating on the back of the lens and further reflects to focus on the cell. The second beam is the angle beam and the combination of the lens, the mirror and the polarizing beam splitter coating constitutes a system that ends up focusing the second beam on the surface of the target cell. This combination acts as a collimator. The cell cooperates with the second beam to measure the angle of the target axis relative to the second beam. The first beam, on the other hand, goes straight through the lens and is partially focused. The first beam and the cell act as a center measuring device and provides signals indicative of X-axis and Y-axis (horizontal and vertical) position or displacement. If the target is moved laterally or perpendicular to the first beam, then position information is read from the target cell substantially as in prior art targets. The problem with the system as described above is that the partial focusing means also is partially sensitive to angle. A conventional target with no lens measures center, but is not sensitive to angle. However, in the subject invention, due to the fact that angle is measured, the center reading can be corrected for any angle.
The primary advantage of this latter embodiment is that the combination of the first beam and the lens of the cell act such that spot size is reduced in diameter. This effectively increases the cell size, and hence the range of measurement is increased. The amount of the increase is proportional to the ratio of the focal length of the lens divided by the distance from the nodal point of the lens to the surface of the cell. As noted above, position sensitive target cells can be expensive, especially for larger sizes. Smaller cells, however, are relatively inexpensive. Thus, an advantage of the system described above is that a small cell can behave as if it were physically much larger by the ratio of the two distances, as long as resolution is adequate. Thus, the above-described combination uses a very economical method of achieving larger measurement ranges.
The second beam is focused onto the photosensitive target, as described above. In particular, the lens of the above-described system is a piano-concave lens with a polarizing sensitive 50%xe2x80x9450% beam splitter coating on the plane or flat side of the lens. A polarizing sensitive coating will transmit almost 100% of P polarization and almost totally reflect S polarization. The plane of polarization of both the first and second beams are rotated so that approximately 100% of their light is transmitted. In this embodiment, a quarter wave plate is aluminum, Al, coated on the back to form a mangrin mirror. A quarter wave plate will take a plane polarized beam and turn it into a circularly polarized beam. Upon reflection, the beam will change xe2x80x9chandxe2x80x9d from right circularly polarized to left hand. As the second beam passes again through the quarter wave plate, it is changed from circularly polarized light back to linear, but is rotated 90xc2x0. The second beam then reflects 100% off the back of the lens and focuses on the lens.